In order to illustrate the prior art technique, reference can be made to FIGS. 1 and 2 of the accompanying drawings. In FIG. 1 is shown, in frontal elevation, a detail of a known rollable grating, and in FIG. 2, at a larger scale and in a vertical cross-section, one of the ties used for the connection of the bends or elbows of the wavy tubes 10 and the horizontal tubes 12 forming the grating. In FIG. 2, one sees that the tie 14 which closes the assembly formed by two wavy tubes and a horizontal tube is provided in the form of a metal-strip with a fastening lug 16.
Usually, such gratings and their mode of connection is generally satisfactory. However, they have the disadvantages of being relatively heavy and having to be periodically painted. It is quite obvious that such painting operations, repeated several times, do not favour the operation of the system. On the other hand, even a painting made on galvanized parts provides only an imperfect protection against corrosion. Finally, the weight of the steel grating requires relatively heavy control members.